1. Field of the Disclosure
The present disclosure relates generally to electrophotographic image forming devices, and more particularly to a photoconductor having protective overcoat layer with a charge transport molecule with four radical polymerizable hydrophilic functional groups containing an oxygen atom and method of making the same.
2. Description of the Related Art
Organic photoconductor drums have generally replaced inorganic photoconductor drums in electrophotographic image forming device including copiers, facsimiles and laser printers due to their performance and advantages. These advantages include improved optical properties such as having a wide range of light absorbing wavelengths, improved electrical properties such as having high sensitivity and stable chargeability, availability of materials, good manufacturability, low cost, and low toxicity.
While the performance and advantages offered by organic photoconductor drums are significant, inorganic photoconductor drums offer much higher durability. Inorganic photoconductor drums (e.g., amorphous silicon photoconductor drums) are ceramic-based, thus being extremely hard and abrasion resistant. The surface of organic photoconductor drums are typically comprised of a low molecular weight charge transport material, and an inert polymeric binder. Therefore, the failure mechanism for organic photoconductor drums typically arises from mechanical abrasion of the surface layer due to repeated use. Abrasion of photoconductor drum surface may arise from its interaction with print media (e.g. paper), paper dust, or other components of the electrophotographic image forming device.
The abrasion of photoconductor drum surface degrades its electrical properties, such as sensitivity and charging properties. Electrical degradation results in poor image quality, such as lower optical density, and background fouling. When a photoconductor drum is locally abraded, images often have black toner bands due to the inability to hold charge in the thinner regions. This black banding often marks the end of the life of the photoconductor drum.
Increasing the life of the photoconductor drum will allow the photoconductor drum to become a permanent part of the electrophotographic image forming device. In other words, the photoconductor drum will no longer be a replaceable unit nor be viewed as a consumable. Photoconductor drums with a life-of-the-printer will allow the printer to operate with lower cost-per-page, more stable image quality, and less waste.
To achieve a long life photoconductor drum, especially with organic photoconductor drum, a protective overcoat layer may be coated onto the surface of the photoconductor drum. While a robust overcoat layer improves the life of photoconductor drums, a suitable overcoat layer is required that does not significantly alter the electrophotographic properties of the photoconductor drum. If the overcoat layer is too electrically insulating, the photoconductor drum will not discharge and will result in a poor latent image. On the other hand, if the overcoat layer is too electrically conducting, then the electrostatic latent image will spread resulting in a blurred image. Thus, a protective overcoat layer that improves life of the photoconductor drum must also allow charge migration to the photoconductor surface for development of the latent image with toner.
Incorporated within overcoats are crosslinkable charge transport molecules. Many prior art overcoats use crosslinkable charge transport molecules having hydrophobic radical polymerizable functional groups. For example, U.S. Pat. No. 9,005,855 to Iwadate teaches that it is desirable for an overcoat to use charge transport molecules having hydrophobic radical polymerizable functional groups as opposed to hydrophilic radical polymerizable functional groups. Iwadate reasons that it is desirable to use a hydrophobic functional group in a charge transport molecule because when hydrophilic or water loving functional groups are used in a charge transport molecule, layer separation in the photoconductor and accordingly deterioration of the efficiency in the charge transport occurs. Additionally, partial moisture absorption naturally caused by using a hydrophilic functional groups in a charge transport molecule leads to a decrease in environmental stability for the photoconductor.
An important feature of the present invention is incorporation of an oxygen atom in the radical polymerizable functional group. The presence of the oxygen atom makes these functional groups hydrophilic by virtue of their ability to hydrogen bond with water. More importantly, radical polymerizable functional groups containing an oxygen atom, such as the acrylate group, methacrylate group, glycidyl ether group and epoxy group, are believed to have higher reactivity towards radical polymerization than olefinic hydrophobic radical polymerizable functional groups such as the styrenic group, the allylic group or the vinylic group. The overcoat of the present invention using charge transport molecules having hydrophilic functional groups as opposed to hydrophobic functional groups, surprisingly leads to a photoconductor having no layer separation, excellent hole mobility and robust mechanical properties.